BSI IC-Gel is great stuff! (I assume you are only gluing frags. Large pieces need epoxy.)
A few things that will help-
1. Look for a rough spot where you can glue the frag. If you try to glue to a smooth area, like on coraline the glue has a harder time of staying. The rough areas allow the glue something to gain purchase on.
2. Take a tooth brush to the area of rock you are gluing to scrub away the slime and detritus.
3. Use large globs of glue. As you have noticed, when you place the glue in the water it forms a film around glob. So, once you place the piece down, work it around on the rock for a moment to break the film bubble that formed around the glue. This allows some of the fresh glue to release onto the rock.
Good luck, hope this helps!